You'll have to change the Model Number in the clodeplug of the XTL5000 over to an XTL2500 and then upgrade the radio with matching firmware. There's no specific need
to change the FlashCode unless there are additonal features that you desire to add into the mix. Use the XTL's 26-Pin Accessory connector to push in the codeplug chage
and to upgrade (change) the firmware. The remote control head should also receive the necessary firmware via the brick during this process.

Firmware upgrades should be done over serial cable to the back of the radio.

Save tuning information. After the model change the model in the tuning info will have to be updated to match. Test this first.

Only upgrade firmware as far as v20.50.07

Change model number to match the control head?

Don't use the CH synch file on an XTL1500. Change radio model first?

Odyssey heads are a separate entity with their own processor onboard, which has its own firmware, so continuing the above analogy the radio and CH are like two different PCs communicating over a network, you need to "sync" them up so they can talk to each other.

The model number will tell you what the control head should

EX: D04UJH9SW7AN Denotes that a W7 head is what radio is configured for. EX: D04UJF9PW4AN denotes that it is configured for a W4 head.

The 9th and 10th digits in model will tell you what the radio control head was when it left the factory.

TIB= transmitter interface board. The Astro Spectras remote mount radio will have 2 db25 connectors on the TIB and also the XTL series that uses the W Series control heads.

If the seller can show you the TIB part number , it would tell you what kind of CH it can use.
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HLN6915 is for the HHCH W3.

HLN6883 is the W4,W5, W7 and W9 heads.